Variation in the strength of diversity effects among experimental studies raise the question when and where consequences of diversity loss is strongest. As in grassland experiments, diversity effects on plant biomass production can be observed in systems with marine macroalgae. However, even among marine macroalgae experiments variation in the strength of the diversity effect cannot be explained because of largely differing experimental set-ups (i.e. long-termed vs. short-termed studies, mesocosms vs. field experiments, using inter- or subtidal habitats). From literature Stachowicz et al. assumed that short termed factors regulating diversity effects in such systems could be attributed to spatial complementarity in photosynthesis rates or different limiting nutrients. Long-term regulating factors could be attributed to habitat differentiation, temporal complementarity, fascilitation, recruitment and natural heterogeneity of substrate. In a very elegant way Stachowicz and his co-workers tested whether mechanisms responsible for diversity effects change with experimental procedure and/or study type within the same marine algae system. In a series of three experiments, that is a short-termed mesocosm with transplanted thalli, a short-termed (two month) field-experiment with naturally recruited thalli and heterogeneous substrate, and in a long-term (three years) field-experiment, the authors were able to show that strong diversity effects are positively correlated with experimental duration, environmental heterogeneity and population responses (recruitment). Whereas in the mesocosm species identity affected biomass production, in the field studies it was species richness but not identity. Fractional change of species biomass could be explained by species identity in the mesocosm, and by both identity and richness in the field. The authors are making an important point by showing that mechanisms for diversity effects are not exclusive but occur together and become stronger over time. They conclude that the absence or the detection of only weak diversity effects in short-termed experiments does not necessarily mean that there is no effect because such approaches detect only a limited number of potential mechanisms.
John J. Stachowicz, Rebecca J. Best, Matthew E. S. Bracken, Michael H. Graham (2008). Complementarity in marine biodiversity manipulations: Reconciling divergent evidence from field and mesocosm experiments. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0806425105